Total luminous flux is one of the key parameters describing the net lighting performance of a light source, which is defined as the sum of luminous flux [unit: 1 m] emitted by a light source in all directions. For example, luminous efficacy [unit: 1 m/W] is evaluated based on the total luminous flux, which is determined as a ratio of an output total luminous flux [unit: 1 m] to an input electrical power [unit: W]. Accordingly, accurate measurement of a total luminous flux is very important in evaluating the performance of lighting apparatuses.
In general, the measurement of a total luminous flux is made by a gonio-photometer. After measuring the spatial distribution of an output luminous intensity over 4π solid angle using a reference photometer of which luminous intensity responsivity has been calibrated, the total luminous flux is obtained by mathematically integrating the luminous intensity distribution.
Alternatively, the measurement of a total luminous flux can be made by an integrating sphere photometer. In principle, the integrating sphere photometer structurally integrates the luminous flux inside the integrating sphere, and gives an output signal nearly proportional to the total luminous flux value of an light source inside the integrating sphere. Since the integrating sphere photometer utilizes the proportionality, the measurement is performed by comparing a light source to be tested with a reference lamp of which total luminous flux has been calibrated. The integrating sphere photometer is advantageous in simple structure and short measuring time.
In the case that a reference lamp and a light source to be tested are identical in kind, the integrating sphere photometer may easily obtain a highly accurate total luminous flux through a simple comparison measurement. Due to this advantage, the integrating sphere photometer has been widely used in practical works.
However, in the case that a reference lamp and a light source to be tested are different in shape, spectral distribution, and spatial distribution, all the differences make measurement errors. To alleviate the errors, the integrating sphere photometer must be additionally subjected to a correction step. The correction step may include self-absorption mismatch correction, spectral mismatch correction, and spatial mismatch correction.
A reference lamp is used for comparison measurement at the integrating sphere photometer. An incandescent lamp has been conventionally used as the reference lamp. The correction step for a light source to be tested, which is not very different in shape and size from the reference lamp, is already well known in the art.
However, in the case that the light source to be tested is a large area surface light source, self-screening effect may occur to cause difficulty in measuring the total luminous flux using the integrating sphere photometer.